CHLOROPLAST DIVISION IN PLANTS
The plastids of plant cells comprise an essential, metabolically diverse group of eukaryotic organelles. The most well-studied plastids are the green chloroplasts, which proliferate in leaf cells and carry out the life-sustaining process of photosynthesis. Plastids also synthesize many other compounds critical for plant growth and development, including membrane lipids, amino acids and growth regulators, and consequently must be partitioned to daughter cells during cell division. Further, specialized plastid types manufacture products of major agricultural importance, such as oil and starch important for both food and biofuels production. Recently, chloroplasts have also been exploited as factories for the production of biopharmaceuticals. Plastid propagation in dividing cells and the developmentally regulated proliferation of specific plastid types in different plant organs depend on the process of plastid division. Thus, plastid division is a fundamentally critical aspect of plant development with important implications for agriculture and biotechnology.
Our research centers on elucidating the mechanisms powering plastid division in plant cells. This process is orchestrated by a dynamic macromolecular machine composed of several ring-shaped sub-complexes that function in concert to constrict the organelle (Fig. 1A). The recruitment, assembly and biochemical activities of these subcomplexes must be coordinated across the two envelope membranes to achieve organelle fission (Fig. 1B). We are combining the powerful genetic and genomic resources of Arabidopsis with the tools of biochemistry and cell biology to identify the components of the plastid division machinery, define their functions within the division complex, and discover how chloroplast division is regulated. Because key features of plastid division are derived from the cell division machinery in the cyanobacterial endosymbiont from which chloroplasts evolved, we have also strategically incorporated experiments on cyanobacterial cell division into our work, and are pursuing comparative genomic and other computational strategies to uncover the full network of genes and proteins controlling plastid division in plants.
A sampling of chloroplast division proteins studied in our laboratory (all nuclear-encoded):
FtsZ1 and FtsZ2: Tubulin-like proteins related to bacterial FtsZ, a cytoskeletal GTPase that forms a ring at the midcell during bacterial cytokinesis (reviewed by Erickson et al., 2010, Microbiol. Mol. Biol. Rev. 74:504). In plants, FtsZ1 and FtsZ2 are endosymbiotic in origin and colocalize to a ring (the Z-ring) at the chloroplast division site inside the chloroplast stroma (Fig. 2). FtsZ1 and FtsZ2 interact with different assembly regulators and both proteins are required for full plastid-division activity (Schmitz et al., 2009). Mutations that reduce the levels of either FtsZ1 or FtsZ2 protein cause dose-dependent defects in chloroplast division (Fig. 3), suggesting their stoichiometry is important for their in vivo activities. Consistent with this hypothesis, we have recently found that FtsZ1 and FtsZ2 preferentially coassemble in vitro as heteropolymers (Olson, Wang et al., 2010). The plastidic FtsZ ring probably functions both to constrict the inner envelope membrane and to position other components of the division complex.
ARC5*: A member of the dynamin family of large GTPases, which oligomerize to function as
membrane “pinchases.” ARC5 is localized at the division site on the cytosolic surface of the outer envelope membrane where it constricts chloroplasts from the outside. Plants with mutations in ARC5 exhibit enlarged, dumbbell-shaped chloroplasts (Fig. 4). ARC5 has no obvious homologues in bacteria, indicating it is eukaryotic in origin.
*ARC5 is also called DRP5B.
PDV1 and PDV2: Plant-specific proteins of the chloroplast outer envelope that recruit ARC5 from the
cytosol to the surface of the chloroplast at the division site. PDV1 localizes to a punctate mid-plastid ring prior to and during constriction and persists as a spot at the pole of one of the daughter chloroplasts following division (Fig. 5). PDV2 also localizes to the chloroplast division site, but forms a continuous ring and is not retained at the pole following division (Glynn et al., 2008).
ARC6 and PARC6: Proteins of the chloroplast inner envelope that both regulate FtsZ ring assembly and dynamics inside the chloroplast (Fig. 5) and position PDV1 and PDV2 to the mid-plastid division site in the outer envelope (Fig. 6), resulting in ARC5 recruitment to the chloroplast surface. ARC6 is cyanobacterial in origin whereas its paralogue PARC6 is unique to vascular plants. ARC6 and PARC6 have divergent functions, but both play critical roles in coordinating the contractile activities of the FtsZ and ARC5 rings across the two envelope membranes (Fig. 1B)
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